12bolt.com has a good variety of build parts including several different cams, some pistons, rods, valves, etc. I went through him for the majority of my build... he has a wealth of knowledge and would give you good info if you reached out to him.
As far as the propane pistons mentioned above, when I did my rebuild earlier this year, those were scarce and not currently produced so prices when found were quite high, almost to the point of what new aftermarket high compression pistons were.
